A ring-shaped folded and drawn formed portion is typically formed by press forming such as drawing using a surface treated metal plate such as a coated steel plate as a raw material. For example, in PTL 1, a ring-shaped oil groove 17 is formed by a folding and drawing process in a portion of a housing 1 of an electric motor. The folded and drawn formed portion is a portion which is formed by folding a single sheet member, and includes an inner peripheral wall, an outer peripheral wall, and a fold portion which links the front ends of the inner peripheral wall and the outer peripheral wall. When the folded and drawn formed portion requires particularly high dimensional precision, ironing is implemented on the folded and drawn formed portion after the folded and drawn formed portion has been formed. Ironing is a process in which a clearance between a punch and a die is set to be narrower than a thickness of the folded and drawn formed portion prior to ironing, and the folded and drawn formed portion is then ironed using the punch and the die so that the thickness of the folded and drawn formed portion matches the clearance between the punch and the die. Ironing for a folded and drawn formed portion of this kind is also known as a “restrike”.
The folded and drawn formed portion is formed by a mold which is generally configured in the following manner. In other words, a conventional mold is provided with a punch, a die, and a counter pad part. The punch is configured as a columnar member, and the die is configured as a ring-shaped member which is disposed on the outer periphery of the punch. A pushing hole into which the folded and drawn formed portion is pushed is formed between the punch and the die. The die has a shoulder portion disposed on an outer edge of an inlet of the pushing hole and constituted by a curved surface having a predetermined curvature radius, and an inner peripheral surface which extends linearly parallel to the pushing direction from a radius end of the shoulder portion. The outer peripheral surface of the punch and the inner peripheral surface of the pushing hole extend mutually in parallel along the pushing direction of the folded and drawn formed portion.
The counter pad part is a member which is arranged facing the punch and the die in such a manner that the folded and drawn formed portion is positioned between the punch and the die, and the counter pad part pushes the folded and drawn formed portion into the pushing hole by relative displacement of the counter pad part with respect to the punch and the die. The wall surface of the outer peripheral wall of the folded and drawn formed portion is ironed by the shoulder portion when pushed into the pushing hole, and the whole folded and drawn formed portion is gradually thinned until coinciding with the width of the clearance between the outer peripheral surface of the punch and the inner peripheral surface of the pushing hole.